The micro-grid is one of the key technologies of the smart grid that can effectively reduce the impact on the main network and users due to the dispersed location, diverse forms, and characteristics of distributed power sources, and realize effective integration and efficient utilization of distributed power sources. As a result, the micro-grid has received widespread attention in recent years. The operation of the micro-grid is flexible, as it can operate in parallel to large grid or in island.
The micro-grid contains a large amount of renewable energy, and most of the renewable energy is incorporated into the micro-grid through inverters, therefore, it is especially important to control inverters with advanced technology. When the micro-grid is in grid-connected operation state, each inverter needs to accurately transmit or receive active and reactive power according to power commands issued by the central controller of the micro-grid, and thereby meets requirements of specific power exchange between the micro-grid and large grid and improves energy management efficiency within the micro-grid.
The micro-grid of medium- and low-voltage grades, however, is often at the end of the distribution network. The voltage quality of the common connection with the distribution network is not ideal. There are often numerous background harmonics, which seriously affects the power transmission quality of the grid-connected inverters. At the same time, sag and swell of the grid voltage amplitude, frequency fluctuations, and line impedance changes cause the inverter transmission power transmitted by the inverter to fluctuate, which deviates from the preset power command, resulting in the decrease in energy transfer efficiency and even leading to negative effect on the system stability and inverter integration.
At present, the control of the transmission power of the grid-connected inverter is mainly realized by controlling the output current of the inverter. The traditional control method is mainly double-loop control. The outer loop controller generates the reference signal. The inner loop is mostly the current loop, which is related to the stability precision, harmonic content, dynamic response, and anti-disturbance ability of the inverter. According to different coordinate system selection, different control strategies are needed. For example, in the synchronous rotating coordinate system, AC variables need to be converted into DC variables, and the proportional integral controller (PI) is used to eliminate the steady-state error. However, in the non-ideal state of the grid voltage, variables obtained by the synchronous rotating coordinate system conversion is no longer DC variables, and traditional PI control is not able to meet requirements of zero steady-state error tracking. And the control in the synchronous rotating coordinate system involves phase-locking part. The performance of phase-locking part directly affects the dynamic response speed of the system and the control effect of the inverter. If the control is performed in the two-phase static coordinate system, the synchronization part required for the transformation can be eliminated, and the complexity of the control system can be reduced. The proportional integral regulator (PR) can exhibit high gain characteristics at the fundamental frequency, which can approximatively achieve error tracking for a sinusoidal quantity of a specific frequency. However, when the sinusoidal component contains other harmonic components, the resonance controller has to be incorporated at specific frequency to eliminate other harmonic components, which increases the complexity of the controller. In the control under the three-phase natural coordinate system, the three-phase system is divided into three single-phase systems for control, thus the control of each phase is relatively independent. Although it can be used for three-phase unbalanced systems, the structure of the controller is relatively complicated.
At the same time, with the in-depth study of nonlinear control theory, some nonlinear control performance characteristics such as fast dynamic response, global stability, and strong robustness, etc., to some extent, make up for the lack of linear control. In recent years, the characteristics have attracted more and more scholars' attention, and have also been applied in the control of inverters, but there are still many problems need to be solved.